Location based services require a good knowledge of the geographical position of users, outdoors as well as indoors. A user's geographical position may be determined using a Global Positioning System (GPS), or more broadly Global Navigation Satellite System (GNSS), receiver of the user's portable electronic device, Outdoors, and in areas of good GPS signal reception, the location can typically be obtained, potentially with an accuracy of approximately 5 to 10 meters.
Positioning services are often unavailable indoors, or in areas of poor GNSS signal reception. In such cases the user has to be localized via other means like assisted GPS systems that use cellular or Wi-Fi radio signal strength indicator (RSSI) measurements to provide positioning information. Current techniques for position determination without GNSS using cellular or Wi-Fi measurements rely on a simple technique that uses a centroid of location fixes of previous users who have seen the same cellular tower or Wi-Fi access point. If a user sees multiple towers or access points, the positioning technique typically only uses the dominant tower or access point. That is, if multiple towers or access points are visible, only the tower or the access point having the strongest signal strength is used in determining the location. The resulting localization accuracy with cellular radio is typically relatively poor, for example approximately 1000 m.
One approach to generalize the above technique in order to incorporate information from multiple visible towers or access points is to assume a simple geometric path-loss model for radio propagation. The signal strength may then be used as a weighting factor in calculating a weighted centroid using the known locations associated with the visible towers or access points. However, such a model is typically very simplistic and cannot capture the complexities of multi-path, fading and occlusions, as a result the localization accuracy still remains relatively poor, for example in the order of 600 m.
It is desirable to have an improved technique of determining an estimate of a mobile's location based on information from multiple towers or access points.